The present invention described herein pertains generally to miniemulsion polymerization processes in which an oil-modified polyurethane resin is dissolved in a vinyl monomer or monomer mixture which is then dispersed under high shear conditions, in the presence of at least one surfactant and optionally, a non-reactive cosurfactant, into an emulsion of submicron droplets. The emulsion is polymerized by a free radical mechanism under conditions which favor nucleation of the monomer droplets. The resultant submicron polymer particles contain polymer with oil-modified polyurethane resin grafted to the backbone polymer.
Water-based coatings, and polymer latexes in particular, have become more widely used in the past several decades because they are environmentally friendly, offer easier clean up, and offer improved performance characteristics. However, in spite of these advantages, solvent based, oil-modified polyurethanes have remained important for some applications because of superior properties such as gloss, chemical resistance and film formation.
Emulsion polymerization is a widely used technique which has been extensively described in literature, both patent and non-patent. Production of synthetic latexes via emulsion polymerization is well-known. Among the polymers commonly produced by emulsion polymerization are styrene-butadiene copolymers, acrylic polymers and copolymers, and polyvinyl acetate. Polymers prepared by emulsion polymerization are widely used as binders in water-based latex paints for both interior and exterior use. Emulsion polymerization is also used to prepare polymer foams and polymers used as coatings.
Emulsion polymerization requires the following key ingredients: water, a monomer or mixture thereof, a surfactant or mixture thereof, and a polymerization initiator. The monomer or mixture thereof is typically dispersed into droplets and polymer particles are formed during the polymerization with the aid of a surfactant or mixture thereof with the aid of an agitator. Monomer droplet diameters are typically from 1 to 10 microns.
Emulsion and miniemulsion polymerizations have many similarities but the particle nucleation and reagent transport phenomena are very different. Conventional emulsion polymerization starts with a monomer emulsion comprised of relatively large (in the range of 1 to 10 microns) monomer droplets and significant free or micellar emulsifier. Particle nucleation takes place early in the reaction via homogeneous (water phase) reactions or via free radical entry into monomer-swollen micelles. Radicals can enter the monomer droplets but this phenomenon is generally discounted because of the relatively small droplet surface area. Nucleation stops or slows significantly after the surface area of the particles becomes sufficient to adsorb all of the emulsifier. The major locus of polymeriation thereafter is in the nucleated particles. The reagents (monomer, chain transfer agents, etc.) must move from the monomer droplets to the reaction sites in the particles. Transport of large hydrophobic molecules such as oil-modified polyurethane can be a problem in conventional emulsion polymerization.
Miniemulsion polymerization, by contrast, begins with submicron droplets which are able to accommodate most of the added emulsifier. High intensity fluid deformation and a cosurfactant are employed to generate and stabilize the small droplet size miniemulsion. Particle nucleation is primarily via droplet penetration and, if most droplets are nucleated, the reagents are located at the polymerization sites and mass transport, except for the radicals, is not involved. Either water-soluble or oil-soluble initiators can be employed in miniemulsion polymerization.
Monomer droplet size instability is observed in monomer emulsions. The smaller monomer droplets will disappear by two mechanisms. The first is flocculation into larger droplets. This can be effectively prevented by providing an adequate layer of surfactant at the droplet surface. The second is Ostwald ripening. This phenomenon consists of the diffusion of monomer out of the smaller droplets and into the larger ones; the polymer does not so diffuse. The net effect is a reduction in interfacial surface area, and hence, of surface free energy. In an unpolymerized conventional emulsion (which will be called herein a xe2x80x9cmacroemulsionxe2x80x9d), the disappearance of the small droplets takes place in seconds. This precludes the nucleation of these droplets into polymer particles. In a miniemulsion, a combination of high shear and a cosurfactant are used. The high shear generates very small monomer droplets. The cosurfactant retards Ostwald ripening so that the small droplets can resist diffusional instability. The small droplets can then compete effectively for water-borne free radicals, and the locus of nucleation becomes predominantly the monomer droplets. Common cosurfactants include hexadecane and cetyl alcohol.
U.S. Pat. No. 5,686,518 discloses the use of monomer-soluble polymers as cosurfactants in miniemulsion polymerization, wherein the monomer-soluble polymers are non-reactive with the monomer polymerization and are effective as cosurfactants at a concentration of about 0.5 to about 5.0 percent by weight based on monomer plus polymer. The monomer polymerizations disclosed therein exhibited reaction rates slightly enhanced with increasing concentration of the non-reactive, monomer-soluble polymer cosurfactants.
In accordance with one aspect of the present invention, there is provided a process for miniemulsion polymerization of a monomer or mixture thereof in the presence of an oil-modified polyurethane resin, which comprises dissolving one or more oil-modified polyurethane resins having at least two carbon-carbon double bonds, in an ethylenically unsaturated monomer or mixture thereof, thereby forming a solution, and combining this solution with water and at least one surfactant, and agitating the resulting mixture. The monomer or mixture thereof is substantially insoluble in water and the one or more oil-modified polyurethane resins are substantially insoluble in water but soluble in the monomer or mixture thereof;
The amount of the one or more oil-modified polyurethane resins is from about 10 percent to about 120 percent based on monomer weight; thereby obtaining an essentially stable aqueous monomer emulsion comprising an aqueous continuous phase and an organic disperse phase. The disperse phase comprises a monomer or mixture thereof and one or more oil-based polyurethane resins, and is in the form of droplets having an average droplet diameter in the range from about 10 to about 1,000 nanometers.
A non-reactive cosurfactant, including, but not limited to, hexadecane, cetyl alcohol, or polymer soluble in said monomer or mixture thereof, may be added to the monomer/oil-based polyurethane solution in the amount of 1 to 5 percent by weight based on monomer to impart diffusional stability to the emulsion.
The emulsion is subjected to high rates of shear in a sonicator, homogenizer, colloid mill or other device capable of imparting rates of shear great enough to reduce the diameters of the droplets of the disperse phase to the range listed above.
This invention according to a further aspect provides a process which comprises: forming an aqueous monomer/oil-modified polyurethane resin emulsion by combining at least one ethylenically unsaturated monomer with at least one oil-modified polyurethane resin having at least two carbon-carbon double bonds; agitating the emulsion under high shear to form monomer particles having an average particle size in the range of about 10 to about 1,000 nanometers; and initiating the polymerization by adding a free radical initiator, thereby polymerizing the at least one ethylenically unsaturated monomer in the emulsion and forming a polymer emulsion in which the polymer content is in the form of particles having an average particle size in the range from about 10 to about 1,000 nanometers.
This invention according to a still further aspect provides a latex or polymer emulsion which is the product obtained by polymerizing the monomer emulsion by a free radical mechanism under conditions of miniemulsion polymerization. The polymer content is in the form of particles having an average particle diameter in the range from about 10 to about 1,000 nanometers.
These and other objects of this invention will be evident when viewed in light of he drawings, detailed description and appended claims.